This invention relates to the field of biomedical instrumentation and, in particular, to reflectance-mode blood circulation monitors.
Measurements of skin pallor are particularly useful in physiological diagnoses and the monitoring of patients. Changes in skin pallor are recognized as cardinal signs of motion sickness, nausea, and shock, and presumably are due to changes in the volume of blood in the microcirculation of the skin. Assessing these changes in peripheral vascular activity using a light source and a photodetector is known as reflectance-mode photoplethsmography.
Attempts have been made to measure blood circulation through skin, employing tungsten lamps and photoconductive cells but several problems have prevented these devices from producing a signal proportional to the total blood volume. Temperature sensitive photocell detectors produce a hysteresis related to prior light exposure. Additionally, relative motion between the transducer and the skin generates artifactual outputs as a result of distance variations between the transducer and skin or compression of the dermal blood vessels. As a result, prior art devices have measured only blood volume pulse amplitude (BVP) rather than absolute reflectance (pallor). Moreover, when visible light is utilized, the BVP devices typically cannot be used on dark skinned individuals and artifactual light components (arising from the ambient environment) seriously degrade the signal.
The deficiencies of the tungsten lamp/photocell designs have led designers to suggest the use of solid-state infrared LED/silicon phototransistor and photodiode devices as photoplethysmographic instruments. Unfortunately, such devices also are sensitive to the infrared components of room and natural lighting. Moreover, the LED light source output can vary with temperature, and motion artifacts can still be present.
Devices which can accurately measure skin pallor will find applications in a variety of cardiovascular and neurological diagnoses. In particular, devices which can provide monitoring of either acute or ambulatory illnesses by detecting changes in skin pallor can satisfy a long-felt need. Additionally, such devices can be useful in detecting and analyzing particular circulatory ailments, such as Raynaud's syndrome which apparently is caused by occupational exposure to vibration and results in reduced vascular circulation in the extremities of those afflicted.
Therefore, there exists a need for devices to monitor skin pallor and preferably, skin temperature, BVP and heart rate as well. Such devices should provide ambient light cancellation and compensate for temperature variations in the LED light source. Such devices should also minimize motion artifacts.